Bombesin (ID#2) is a 14 amino acid peptide, originally isolated from the skin of the frog Bombina bombina. Bombesin is also structurally related to a number of other peptides including Gastrin Releasing Peptide (ID#1), and Litorin (ID#3) (See Sequence identification Section).
Bombesin is known to have a range of effects including stimulation of the nervous system, reduction of renal blood flow, secretion of pituitary hormones, growth promotion, memory retention, induction of myoelectric and contractile activity of intestinal myocytes, induction of gastric and pancreatic secretion, and bolstering of immune function. There has been considerable interest modulating these activities in the design and development of bombesin analogs as possible mimics or inhibitors of bombesin action in the body.
The bombesin-dependent responses occur through a class of high-affinity (KD=1 nm) cell surface receptors that bind bombesin. Binding of Bombesin to its cell surface receptor elicts cell mitogenic responses in a number of tissues. The mitogenic response has been demonstrated in a number of cell types including Swiss 3T3 embryo fibroblast cells, human bronchial epithelial cells, human small cell lung carcinoma cells, rat gastrin cells , and rat pancratic cells. Similarly, Bombesin induction of gastric and pancreatic secretions, important for digestive functions, occur through the receptors found on cells of pancreatic (B-Cells) and intestinal gastrin cells (G-cells).
Binding of Bombesin to its extracellular receptor evokes a number of intracellular signals including activation of G-proteins, which in turn activates phospholipase C (PLC). PLC in turn converts phosphatidylinositol phosphate (PI) into inositol 1,4,5,-triphosphate (IP.sub.3) and diacylglycerol (DAG). IP.sub.3 and DAG are believed to be intracellular signals for cellular mediated events.
Structure-activity studies indicate that receptor-binding requires a peptide ligand containing an amidated C-terminal, and generally the presence of the last eight amino acids. Recent work has concentrated on modifying the carboxy terminal (C-terminal) region of Bombesin to selectively modulated the receptor interaction utilizing a variety of different types of C-terminal modified analogs. These modifications have included, for example, incorporation of D-amino acids, non-peptide bonds, amide, and ester modifications. These alterations have given rise to certain peptides having improved characteristics.
The applicants have prepared linear peptide analogs of the natural bombesin containing a non-peptide bond between amino acids 8 and 9, consisting of a methyl sulfide group (.PSI.[CH.sub.2 S(CH.sub.3)]) or a methyl amide group (.PSI.[CH.sub.2 N(CH.sub.3)]). The applicants have demonstrated that these analogs act at the bombesin receptor and elicit or prevent required intracellular signals for cellular response of bombesin. The peptide analogs of this invention potentially possess significant mitotic and/or secretory activity and therefore may allow for a scientifically interesting and therapeutically significant adjunct to growth therapy and/or the treatment of digestive disorders. Moreover, the presence of methyl sulfide and methyl amide functionalities, or des-methionine analogs having D-amino acids and N-terminal modifications may provide for enhanced potency and extended duration of action.